In the field of telecommunications, efforts have recently been directed towards developing advanced multiple access cellular systems. Multiple access refers to the method by which multiple users simultaneously gain access to the system. CDMA is a multiple access method, which is a direct sequence spread spectrum (DS-SS) technique and which has recently been implemented in commercial cellular radio systems. CDMA has several advantages over other methods of multiple access, such as time division multiple access (TDMA). For example, spectral efficiency and the simplicity of frequency planning are improved in CDMA. An example of a CDMA system is described in the publication, Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System, TIA/EIA/IS-95, July 1993.
In CDMA, the narrowband data signal of a user is shifted to a relatively wide band by multiplication with a spreading code having a considerably broader band than the data signal. Bandwidths such as 1.25 MHz, 10 MHz and 25 MHz have been used in CDMA. As a result of the multiplication, the data signal spreads throughout the entire wideband. All users simultaneously transmit using the same frequency band. A separate unique spreading code is used over each transmission channel between a base station and user of a mobile station, and the signals of the different users can be distinguished from one another in the receivers based on the unique spreading codes.
Matched filters in the receivers are synchronized with a pilot signal, which may be identified based on the unique spreading code assigned to a channel carrying the pilot signal. A transmitted signal is restored in the receiver to the original band by multiplying it by the same spreading code that was used in the transmitter. In an ideal case, signals multiplied by some other spreading code do not correlate and are not restored to the narrow band. They appear thus as noise with respect to the desired signal. The spreading codes of the system are preferably selected in such a way that they are mutually orthogonal, i.e., they do not correlate with each other.
In a CDMA cellular radio system, such as an IS-95 system, a pilot channel is used in the transmission direction of the base station to subscriber equipment, i.e., in the downlink or forward link direction. The pilot channel is a signal that is transmitted with a specific spreading code in the same frequency band as the traffic channels. The pilot signal is distinguishable from the other channels based on the unique spreading code assigned to it. The pilot signal is monitored by all mobile stations within the coverage area of a base station and is used, for example, in power measurements and in the generation of a coherent phase reference. Each base station of the system transmits its own pilot signal, from which the subscriber equipment can distinguish the transmissions of different base stations from one another.
In a typical cellular environment, the signals between a base station and a mobile station propagate along several paths between the transmitter and the receiver. This multipath propagation is mainly due to the reflections of the signals from surrounding object surfaces. Signals which have propagated along different paths arrive at the receiver at different times due to their different transmission delays. The receiver generally utilized in a CDMA system is a multibranch receiver structure in which each branch is synchronized with a signal component which has been propagated along a separate individual path. Each branch is an independent receiver element, the function of which is to compose and demodulate one received signal component. In a conventional CDMA receiver, the signals of the different receiver elements are combined to generate a signal of good quality.
CDMA systems can also apply a soft handover wherein a mobile station may simultaneously communicate with several base stations by utilizing the multibranch receiver's ability to simultaneously decode multiple channels. The connection quality of the mobile station remains high during the handover and the user does not notice a break in the connection.
Typically, a CDMA signal is transmitted in an omnidirectional pattern within a normal cell sector of up to 360 degrees. Commonly, three sectored cells are used where the omnidirectional transmission is over 120 degrees, and interference caused by other transmissions within the desired channel effectively appears in the receiver as noise that is evenly distributed. This is also true when a signal is examined in an angular domain, based to the incoming direction of the signals detected in the receivers. The interference caused by the other channels with a channel also appears in the receiver as distributed in the angular domain, i.e., the interference is rather evenly distributed across the different incoming directions.
A capacity increasing multiple access technique called space division multiple access (SDMA) has recently been developed. This is performed in such a way that the beams of the transmitter antennas at the base station are adjusted to the desired directions according to the location of the mobile stations. For this purpose, the system uses adaptive antenna groups, i.e., phased antennas, and the processing of the received signal by means of which the mobile stations are tracked.
The use of the SDMA and beamforming antennas in connection with CDMA provides several advantages over conventional omnidirectional antennas. SDMA has been proposed as a method to increase the capacity of proposed IS-95 3rd generation systems forward links. Using beamforming antennas on only the forward link allows capacity increase without added mobile complexity. This may, however, result in a resource cost at the base station if the base station tracks the mobile station position. The complexity increase is because multipath searching must be performed in the dimensions (i.e., time and space) and antenna patterns must be changed based upon positional information on the mobile stations. Other changes required to support beamforming antennas on the forward link include the addition of a pilot signal specific to the beamformed antenna called an auxiliary pilot. This auxiliary pilot signal can be provided by imbedding reference symbols in the traffic channel or broadcasting a known channel in parallel to the traffic channel. The increase in capacity of a CDMA cellular system provided by implementing beamforming antennas clearly has a cost.